The invention relates to pyroelectric ceramic material on the basis of lead titanate, the use of this material in pyroelectric detectors for infrared radiation as well as pyroelectric detectors.
Pyroelectric detectors for infrared radiation can suitably be used to detect moving warm objects emitting thermal radiation in the wavelength range from 8 to 12 .mu.m, which corresponds to a body temperature of approximately 300K. Pyroelectric detectors can typically be used, for example, in alarm systems or as replacements for light barriers.
Materials which can suitably be used for pyroelectric detectors should have a polarization which changes as much as possible with temperature. For example, crystals are known which are electrically charged in specific crystal faces as a result of a change in temperature. Pyroelectric crystals are built up of ordered polar atom groups having an accumulative effect and leading to a polarization P of the crystal. As a result thereof, the boundary surfaces of the crystal carry polarization charges. A change in the temperature of the crystal causes the polarization charges at the boundary surfaces of the crystal to change too because the polarization P is governed by temperature. This polarization charge formed by a change in temperature is termed pyroelectricity. Small charges released by a change in temperature can produce an electronic signal after they have been properly amplified.
The most important parameter for the effectiveness of pyroelectric material is the pyroelectric coefficient (p), i.e. the change of the ferroelectric polarization (P) per degree Kelvin (T) in the desired temperature range:
p=.delta.P/.delta.T (unit: 10.sup.-4 C/m.sup.2 K).
The change of the charge resulting from the pyroelectric effect is electronically detected as a change in voltage usually by means of a field effect transistor, at the pyroelectric layer according to the equation U=Q/C, where U is the pyroelectric voltage in Volt, Q is the charge in Coulomb and C is the capacitance of the pyroelectric layer.
To obtain the highest possible pyroelectric voltages, the capacitance C of the pyroelectric layer must be as small as possible, which at a given layer thickness d and a given surface A of ceramic material can only be attained through the smallest possible relative dielectric constant .epsilon..sub.r of the ceramic material, according to the equation: EQU C=.epsilon..sub.0 .times..epsilon..sub.r .times.A/d.
A further important factor for the effectiveness of pyroelectric ceramic material is the so-called "Figure of Merit" (F.sub.M); EQU F.sub.M .apprxeq.p/(.epsilon..sub.r .times.C.sub.p),
where C.sub.p is the thermal capacity.
As the thermal capacity of ceramic sintered bodies having a similar composition changes only little, similar materials are often alternatively evaluated by means of a simplified formula for the Figure of Merit: EQU F'.sub.M .apprxeq.p/.epsilon.r.
Apart from exhibiting the highest possible values for F'.sub.M, ceramics for pyroelectric detectors should, with a view to industrial scale manufacture, be suitable for the manufacture of surface mounted devices (SMD). To solder SMD components, they are passed through a wave solder bath and exposed to temperatures up to approximately 250.degree. C. for a short time. The Curie point of the pyroelectric ceramic material must be correspondingly high to prevent the material from being thermally depolarized during soldering, i.e. it should have a Curie point T.sub.c which is above 250.degree. C.
From GB-PS 1 504 283 it is known to manufacture pyroelectric detectors from a material on the basis of lead zirconate titanate (PZT) which is doped with lanthanum and manganese. However, this material can not advantageously be used for pyroelectric detectors because its relative dielectric constant values of .epsilon..sub.r &gt;1700 are too high. Moreover, PZT does not exhibit an optimum polarization behavior. Pyroelectric components on the basis of PZT are subject to substantial aging and, hence, do not exhibit a sufficiently high stability of the polarization P, which results in an insufficiently high reversibility of P.
In Electronic Ceramics 16 (1985), pp. 43 to 55, a pyroelectric ceramic material on the basis of a mixed crystal of lead timate (PT) with calcium titanate is disclosed, which corresponds to the formula EQU Pb.sub.0,8 Ca.sub.0,2 (Ti.sub.0,96 (Co.sub.0,5 W.sub.0,5).sub.0,04)O.sub.3.
During sintering this material, a Pb.sub.2 WO.sub.5 -containing secondary phase is formed on the grain boundaries, which phase is used as a sintering auxiliary phase. A disadvantage of this composition is that tungsten is enriched in the secondary phase and, consequently, cobalt is enriched in the matrix. Since cobalt is an acceptor, a large number of oxygen vacancies are formed whose high mobility leads to ion conductivity. In thin films and at relatively high temperatures, however, ion conductivity may lead to degradation, which is undesirable for the functioning of the components.
Moreover, for reasons of practical use, it is desirable for pyroelectric detectors to exhibit a satisfactory microphonic stability. This is obtained when the ratio of the electromechanical coupling factors k.sub.p /k.sub.t is as small as possible, i.e. &lt;1, which ratio cannot be attained with the known materials for pyroelectric detectors such as, for example, materials on the basis of PZT.